Printing media are improved if they resist curl and remain flat. This is particularly important with media utilized in the formation of images. Such imaging media include ink jet, thermal dye sublimation imaging, thermal wax dye transfer, electrophotography, and silver halide photography. The formation of a flat base is particularly important for color photographic paper. Present color papers will curl during development and storage. Such curl is thought to be caused by the different properties of the layers of the color paper as it is subjected to the developing and drying processes. The layers in prior art photographic papers that influence image curl are cellulose paper and the gelatin emulsion layer. Humidity changes during storage of color photographs lead to curling, as the humidity changes the physical properties of the paper and gelatin emulsion. There are particular problems with color papers when they are subjected to extended high humidity storage such as at greater than 50% relative humidity. Extremely low humidity of less than 20% relative humidity also will cause photographic papers to curl. Curled photographs are undesirable, as they may not be stacked well, displayed on flat surfaces, or displayed in a pleasing manner in albums or when viewed individually. In color papers there is a need for providing color papers with improved resistance to curl.
Color print papers are usually made of three structural layers under the emulsion; cellulose paper base, and a top and bottom coating of extrusion cast polyethylene. Since the strength properties of polyethylene in this form are poor in comparison to the cellulose paper base, the curl properties or the stiffness of a print in bending is determined exclusively by the cellulose paper base. Because of the alignment of the paper fibers during manufacturing, the cellulose paper base is highly oriented with respect to strength properties in the plane of the paper; for example, the ratio of the elastic modulus in the strongest to weakest direction is often 2:1, with the strongest direction in the MD (machine direction). This is why the shape of a typical polyethylene sample with curl, caused by resisting emulsion shrinkage on one surface at low humidity, is cylindrical with the axis of curl along the MD, so the sample curls in its weakest direction, or directly in the CD (cross direction).
Illustrated in FIG. 1 is a photographic image with typical image curl aligned with the MD. Photographic image 10 has curl aligned with the machine direction 14 and perpendicular with the cross direction 12.
The Technical Association of the Pulp and Paper Industry literature suggests that the MD to CD modulus ratio predicts manufacturing efficiency in conversion processes, optimizing bending stiffness, monitors "draws" and the "jet/wire" ratio. An MSA (major strength angle) of a paper web or biaxially oriented polymer sheets is defined as the angle from the machine direction where the modulus of the paper web or biaxially oriented sheet is at its maximum. For example, a paper web with an MSA of 0 degrees has its modulus maximum aligned with the machine direction. A biaxially oriented polymer sheet with a MSA of 10 degrees has its modulus maximum 10 degrees away from the machine direction. The Technical Association of the Pulp and Paper Industry literature suggests that an MSA outside plus or minus 3 degrees predicts "stack lean", dimensional stability, misregistration in printing due to differences in hygroexpansion, baggy edges, and wrinkles. A MSA outside 5 degrees indicates that the paper making head box is out of tune.
Stiffness in the plane of a sheet can be obtained from a Lorentzen & Wettre TSO gauge. This device can draw a polar plot of stiffness, and it is also capable of estimating the major strength angle (MSA) by using sonic waves traveling though a sample in different directions. The sample may be analyzed repeatedly in a MD or CD pattern to map out the range of variation in the MD/CD profile and MSA.
In the absence of a TSO gauge, a tensile test can be done on a group of samples cut at angles from the MD to obtain the polar values. It is necessary to take a large number of samples to be sure that the proper curve shape is obtained. The polar strength of a material can be modeled by the von Mises multimodal distribution equation below: ##EQU1##
The parameter A is used to scale the size of the ellipsoid, K is a shape factor used in the term JO(K) which is a Bessel function of the first kind and zero order, .THETA. is the angle at which the strength is indicated, and .mu. is the MSA or major axis offset angle.
For assembled laminates, the polar stiffness data may either be elastic modulus readings or bending stiffness data. The bending stiffness of the sheet can be measured by using the LORENTZEN & WETTRE STIFFNESS TESTER, MODEL 16D. The output from this instrument is the force, in millinewtons, required to bend the cantilevered, unclamped end of a clamped sample 20 mm long and 38.1 mm wide at an angle of 15 degrees from the unloaded position. A typical range of stiffness that is suitable for photographic prints is 120 to 300 millinewtons. A stiffness greater than at least 120 millinewtons is required, as the imaging support begins to lose commercial value below that number. Further, imaging supports with stiffness less than 120 millinewtons are difficult to transport in photographic finishing equipment or ink jet printers causing undesirable jams during transport. Supports with an MD stiffness greater than 280 millinewtons will also require too much force to transport a print around some metal guides because the coefficient of friction times the bending force is too high.
To better manage the curl of the photographic paper, replacing the low strength cast polyethylene layers with high strength biaxially oriented polymer sheets is useful. High strength plastic sheets are commonly made by biaxially orienting coextrusion cast thick (1025 .mu.m) polyolefins. The sheets in question may be labeled OPP for oriented polypropylene. Biaxially oriented polymer sheets are typically oriented 5X in the MD and then 8X in the CD. The final major strength properties are aligned with the CD and they are generally 1.8 times that of the MD. The MSA for biaxially oriented sheets can be aligned out of the exact CD direction by 10 degrees or more. For most purposes, a biaxially oriented sheet aligned out of the exact CD direction by 10 degrees or more is of no consequence. An MSA of 10 degrees or more is believed to be related to orientation of the polymer first in the CD and then MD direction.
It has been found previously that to minimize curl in an imaging support material, the elastic modulus for high strength biaxially oriented polymer sheets should be the same order of magnitude as the cellulose paper base. High modulus biaxially oriented sheets, therefore, are superior to the weak polyethylene layers coated on prior art support materials. It has also been found that the primary strength axis for the biaxially oriented sheets should be approximately perpendicular to the cellulose paper base because it is possible to select combinations biaxially oriented sheets adhered to the cellulose paper base to obtain a combined bending stiffness that is equal in the MD and CD direction. It has been previously found that equal bending stiffness in the MD and CD tends to minimize image curl.
It has been found that the condition of equal MD and CD strength is not, in itself, sufficient to keep a laminate from having optimum curling properties. Imaging supports made by laminating biaxially oriented sheets to cellulose paper and having a combined bending stiffness that is equal in the MD and CD direction have been shown to have "diagonal curl" which is curl where the axis of the cylinder of curvature is at an angle between the CD and MD. FIG. 2 is an illustration of diagonal curl or twist warp of a photographic image. Photographic image 20 is curled along the diagonal of cross direction 26 and machine direction 24. Diagonal curl, also known as "twist warp", makes the photographic print appear undesirable because the diagonal direction maximizes the total edge lift when the sample is laid on a table and the curl occurs along the line of maximum photo length. Perceptual testing showed that consumers seem to dislike the diagonal curl, even with small amounts of curl. It would be desirable to reduce diagonal curl of a image.